This application claims priority under 35 U.S.C. 119, and contains subject matter related to Japanese Patent Application No. 11-363059, filed on Dec. 21, 1999, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an electrophoretic display liquid, which reversibly changes its optical properties upon application of an electric field thereto. In addition, the present invention relates to an electrophoretic display medium and device using the display liquid.
2. Discussion of the Background
Electrophoretic display media in which image displaying and image erasing can be performed by applying an electric field thereto while controlling the electric field are well know as reversible image displaying media. Electrophoretic display media (hereinafter referred to as display media) attract considerable attention because of having the following advantages:
(1) images formed in the display media can be used as a memory (i.e., images can be read even when application of an electric field to the display media is stopped);
(2) the display media consume relatively low electricity because the media do not consume electricity when an image displayed is maintained; and
(3) images formed in the display media have an angle of visual field as wide as general printed documents (i.e., the images can be easily observed even at a point such that an angle formed by the point and the normal line of the display plane is large).
Typical electrophoretic display media have a closed cell in which two electrodes, at least one of which is transparent, face each other with one or more spacers therebetween and which includes an electrophoretic display liquid including a dispersion medium and a particulate pigment dispersed in the dispersion medium in which a dye is dissolved and whose color is different from the color of the colored pigment. A white pigment such as titanium oxide or zinc oxide is typically used as the colored pigment, and a liquid such as xylene, perchloroethylene or isoparaffin is typically used as the dispersion medium. In the dispersion medium, a dye having a color largely different from the white pigment, a surfactant and the like material is typically included.
When the opposed electrodes apply an electric field to the display liquid, the white pigment moves toward one of the electrodes of the cell, and thereby the one of the electrodes achieves a white color state and the other electrode achieves a color state whose color is the same as that of the dispersion medium (i.e., the color of the dye). When an opposite electric field is applied thereto, the colored pigment moves toward the opposite electrode, and thereby the opposite electrode achieves a white color state. Therefore, such electrophoretic display media can provide visible images using the color difference between the colored pigment and colored dispersion medium. Japanese Laid-Open Patent Publications Nos.48-71990,48-71991, 48-71992 and 59-165028 have disclosed such display media.
In such display media, the formed images have poor contrast because the color of the pigment is displayed as a mixed color of the pigment and the dispersion medium including a dye. The reason of this problem is considered to be that the dye included in the dispersion medium adsorbs the pigment. Therefore, this problem cannot be avoided in such display media.
In attempting to solve this problem, the following methods have been disclosed:
(1) a titanium coupling agent is included in a colored dispersion medium or coated on a colored pigment (Japanese Laid-Open Patent Publication No. 62-296127);
(2) a display liquid in which a particulate resin dyed with a fluorescent dye and a pigment having a color different from the color of the particulate resin are dispersed in a colorless dispersion medium having a high insulation property and a low viscosity (Japanese Laid-Open Patent Publication No. 62-299824); and (3) organodisilazane is included in a colored dispersion medium having a high insulation property or coated on a colored pigment (Japanese Laid-Open Patent Publication No. 638637).
However, these methods cannot fully improve the problem, and therefore the resultant display media are not practically used because of having poor visibility.
In attempting to solve the above-mentioned problem of the display media including a dyed dispersion medium, display media which do not use a dye solution have been proposed. For example, Japanese Laid-Open Patent Publication No. 62-269124 discloses an electrophoretic display medium including a display liquid in which at least two kinds of electrophoretic particles whose colors and electrophoretic properties are different from each other are dispersed in a colorless dispersion liquid having a high insulation property. Since these two kinds of color particles have different electrophoretic properties (i.e., one has a positive polarity and the other has a negative polarity), the particles tend to aggregate due to electric attraction. Therefore, a problem which occurs is that the resultant image has poor contrast because the image has a mixed color thereof. In addition, Japanese Laid-Open Patent Publication No. 63-50886 discloses a display medium including a display liquid in which at least two kinds of electrophoretic particles, whose colors are different from each other and which have electrophoretic properties such that the charge polarity is the same but the electrophoretic speed is different, are included in a colorless dispersion medium having a high insulation property. Since the medium displays images utilizing the difference in electrophoretic speed between the electrophoretic particles which have the same polarity and therefore move in the same direction, the colors of the particles tend to mix, resulting in formation of images having poor contrast (i.e., poor visibility).
In attempting to solve the problem (i.e., formation of aggregates of different color particles) of the above-mentioned display media including two or more kinds of particles having different colors, a medium including a dielectric display liquid, first particles having a charge and a color, and second particles having a charge whose polarity is different from the polarity of the first particles and which repulse the first particles utilizing steric repulsion effect to avoid aggregation, is disclosed in Japanese Patent Publication Kohyo No. 8-510790. However, it is difficult to perfectly avoid aggregation of the first and second particles by such a method. Therefore, images having good contrast cannot be obtained in the medium.
In addition, Japanese Laid-Open Patent Publication No 10-149117 discloses an ink composition including white particles which includes a resin and a white pigment and which is used for a concealment purpose, colored particles and a solvent. The white particles have a median particle diameter not less than 1.5 xcexcm, and the white pigment is present in the white particles in an amount from 15 to 35% and has a median particle diameter not less then 0.4 xcexcm. In this ink composition, images are formed and erased by transporting one of the white particles and the colored particles using magnetism. The ink composition is suitable for image display devices which display images using magnetism. However, when the ink composition is used for electrophoretic display devices using electricity, the white pigment present in the white particles charges, resulting in formation of aggregates of the white particles and the colored particles. Therefore, the ink composition cannot display images having good visibility, and is not suitable for an electrophoretic display medium.
In addition, Japanese Laid-Open Patent Publication No. 11-119264 discloses a display medium which includes a pair of opposing electrodes in which a plurality of microcapsules including charged particles dispersed in a dispersion medium. The dispersion state of the charged particles is changed by changing the voltage applied to the display medium, resulting in change of the optical reflection properties of the display medium. Thus the display medium performs display operations. In this display medium, the charged particles have a particle diameter of from 1/1000 to about 1/50 of the particle diameter of the microcapsules and a particle diameter ratio, (volume average particle diameter of the charged particles/number average particle diameter thereof), of from 1 to about 2.
In the publication, a display medium is exemplified which includes microcapsules having a particle diameter of 100 xcexcm and including positively-charged white particles having an average particle diameter of 3 xcexcm and a particle diameter ratio of 1.3 and negatively-charged black particles having an average particle diameter of 3 xcexcm and a particle diameter ratio of 1.3.
However, the display device has a drawback such that the black and white particles aggregate due to electric attraction, resulting in formation of display images having poor contrast (i.e., poor visibility).
An electrophoretic display medium in which microcapsules including a colored dispersion medium and an electrophoretic pigment having a different color and dispersed in the dispersion medium are sandwiched by opposing electrodes is disclosed in Japanese Laid-Open Patent Publication No. 1-86116 (i.e., Japanese Patent No. 2551783). Such a display medium has an advantage in that uneven images due to maldistribution of the particles can be avoided. However, the medium has a drawback in that contrast of images is poor because the pigment adsorbs the dye included in the colored dispersion medium.
Because of these reasons, a need exists for an electrophoretic display liquid and medium which can produce images having high contrast and good visibility.
Accordingly, an object of the present invention is to provide an electrophoretic display liquid which can reversibly display images having high contrast and good visibility.
Another object of the present invention is to provide an electrophoretic display medium and device, which can reversibly display images having high contrast and good visibility.
Briefly, these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by an electrophoretic display liquid including at least a dispersion medium, a white particulate material and a colored particulate material having a color different from the white particulate material, wherein the white particulate material and the colored particulate material has a respective volume median particle diameter and wherein the following relationship is satisfied:
1/50xe2x89xa6Dc/Dwxe2x89xa61/1
wherein Dc represents a volume median particle diameter of the colored particulate material and Dw represents a volume median particle diameter of the white particulate material.
The white particulate material is preferably hollow particles having a volume median particle diameter of from 0.2 to 6 xcexcm.
The colored particulate material is preferably titanium black.
The display liquid may be microencapsulated.
In another aspect of the present invention, an electrophoretic display medium is provided which includes a display cell which is constituted of a pair of opposing substrates with one or more spacers therebetween and which includes the above-mentioned display liquid, wherein the pair of substrates includes an electrode at an inside surface thereof and at least one of the pair of substrates is transparent.
Alternatively, the display cell may be formed by a substrate having an electrode formed on the entire surface thereof and an insulating film which faces the electrode with one or more spacers therebetween. In addition, the display cell may be replaced with a display layer including the microencapsulated display liquid and a binder material.
In a further aspect of the present invention, an electrophoretic display device including the electrophoretic display medium mentioned above and a writing device, which can be attached to the other when information is written in the display medium, is provided. The writing device includes an electric field forming device which can apply an electric field to the display medium and which can change its position along the display medium, to display images therein.
These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.